This invention is directed to the coating of disc-shaped material by means of LP CVD processes (low pressure chemical vapor deposition), especially for the processing of semiconductor substrates.
Tube reactors have become especially important with the introduction of chemical vapor-phase separation (CVD) under reduced pressure as well as the method of plasma corrosion. A simple tube reactor is described in DE-AS No. 2460 211 (U.S. Pat. No. 3,900,597). The substrates to be coated are placed centrally in the reactor with their surface normally parallel to the axis of the tube; herein, the gas inlet and the gas outlet are always located at opposite ends of the reactor tube. No special measures are required for guiding the gas. Although this reactor has been used with success for relatively less active processes, it cannot be used for highly active processes where, above all, the risk might be incurred of generating a strong, homogeneous gas-phase reaction that is strongly detrimental to high-quality deposition.
In these processes, the highly reactive gas or the components of a gas mixture must be guided by special means directly into the reaction zone where the substrates are located.
Known solutions (for instance, DE-OS No. 2750 882 or DD WP No. 121975 - normal pressure CVD) comprise guiding the reaction gases by means of thin tubes into the substrate area and from there over jets, in which the gas flow is guided into the intermediate zone between the substrate surfaces.
If various reactive components participate in coat formation, then these may also be guided through separate tubes in the described manner into the substrate area, being only mixed in close proximity to the disc. No special measures are taken to discharge the exhaust gases that have been contaminated with reaction products and particles; they are discharged by a pump connected at the exit of the reactor.
One disadvantage of this solution is that the gas flow is deviated in a tube axis direction to the reactor exit, which produces non-homogeneous coatings which can only be equalized through increased gas flow quantities. A particularly great disadvantage is that the used gases contaminated by reaction by-products and particles after flowing over the substrates, still remain for a certain time in the reaction area, where they flow along the opening between the substrate stacks and the tube wall, also penetrating the substrate intermediate zones. This is a great source of contamination for the substrate, which must be slowed by low concentrations of the processing gases--which results in low deposition rates.
The mentioned normal-pressure-CVD-arrangement (DD WP No. 121975) requires an additional inert gas flow, which is supplied at the reactor inlet, in order to rinse the dead spaces in the reactor and to shorten presence of the gases in the reactor.
DD WP No. 111 935 describes a special mode of gas discharge from a normal-pressure tube reactor. Herein, thin gas-outlet tubes that are provided with openings are arranged in the reactor area parallel to the substrate mountings. In LP CVD processes, the processing gas is suctioned off by vacuum pumps. Waste derived from the suctioning-off operation in the thin, slitted tubes is very large; this could only be compensated through highly over-sized pumps. This measure, however, would not be economical and also not feasible from a technical viewpoint.